The present invention relates to a process for coating electronic elements such as capacitors, resistors, or assembled modules of circuits with powdered coating materials for the purpose of improving their insulation characteristics and protecting the elements from moisture.
More specifically the present invention provides a process comprising the forced impregnation of electronic elements with a liquid thermosetting resin undercoat, applying a powdered coating material thereon as a top-coat by electrostatic fluidized bed dip coating before the undercoat is dry, and curing the resin by heating. The process of this invention provides for many improvements over the prior art processes, such as, for example, reduced working time since it does not require the drying and curing steps of the prior art, improved adhesion of the powdered coating material to the elements, and coatings of uniform thickness and fewer pinholes for better protection against moisture.
The use of powdered coating materials for electronic elements has become popular in recent years due to their superior performance in comparison with conventional solvent-type coating materials, and also due to shortened coating times, an improved working environment, and reduced loss of coating materials. However, conventional coating processes using powdered coating materials are generally associated with certain disadvantages such as (a) insufficient deposition of powdered coating material (leading to uneven thickness), (b) tendency to form pinholes, (c) inferior protection against moisture, and (d) poor adhesion between the coating and the element.
In order to overcome these disadvantages, various coating processes have been proposed. However, such processes still suffer from certain other undesirable features. Some of these processes include the following:
(1) Applying a plasticizer, a thermoplastic resin, or a solvent, etc., to the element as an undercoat, followed by the application of a powdered coating material. This method is undesirable since the coating has a tendency to form pinholes, begin oozing, and show insufficient adhesion to the element.
(2) Applying a thermosetting resin to the element as an undercoat, followed by drying and curing, and by the application of a powdered coating material. This method is undesirable due to poor pickup of powdered coating material, difficulty in obtaining a uniform coating thickness, prolonged working periods, and the possibility of eventual pinhole formation even when a very small amount of coating is applied.
(3) Applying a thermosetting resin to the element as an undercoat, by brush coating or spraying, followed by application of a powdered coating material while the undercoating is still wet. Although this method provides for satisfactory adhesion of the coating to the element, and also for satisfactory pickup of powdered coating material if electrostatic coating methods are employed, the coating has a tendency for pinhole formation resulting in inferior moisture protection.
Some coating processes require a preheating step in which a powdered coating material is applied to a preheated element by such methods as fluidized bed dipping, sprinkling, rolling, and spraying. However, the thermosetting resin undercoat tends to cure during preheating leading to an unsatisfactory adhesion of the powdered coating material to the element and to difficulty in obtaining a uniform thickness. These processes are also disadvantageous in that the powdered coating material is applied to a part of the element where such coating is unnecessary, such as the lead wires of an element.
In order to avoid preheating the element, the coating material can be applied to the element by, for example, dip coating, electrostatic spray coating, and electrostatic fluidized bed dip coating. The dip coating and electrostatic spray coating methods prove to be unsatisfactory, however, for the following reasons. In the dip coating method, only a very limited amount of powdered coating material is picked up by the element even when a liquid thermosetting resin is employed as an undercoat. The electrostatic spray coating method requires protection against the generation of powder dust and is not practical since the elements to be coated are small in size.